Study on the Photocatalytic Degradation of Rhodamine B by g-C3N4/Bi2Fe4O9 Heterojunction Photocatalyst

g-C3N4/Bi2Fe4O9 composite photocatalysts were prepared by the mechanica mixing-calcination method. The heterojunction structure formed by the composite of the two materials made it difficult for photogenerated electrons and holes to reunite and improved the degradation rate of RhB. At 60 min, the adsorption rate of 25%-g-C3N4/Bi2Fe4O9 was higher than that of Bi2Fe4O9 and g-C3N4, and the adsorption rate reached 33.33%. At 240 min, 25%-g-C3N4/Bi2Fe4O9 photocatalytic degradation efficiency of 10 mg/L RhB was good, reaching to 87.59%. The reaction rate constant was 0.00674 min−1, which was 2.53 times and 2.32 times that of Bi2Fe4O9 and g-C3N4. Superoxide radical (O2−·) and ·OH were the main active components in the dye degradation process.


Introduction
With the development of industrialization, environmental problems are becoming more and more serious.Nowadays, a large number of toxic and harmful chemical pollutants are being released into the environment around us, causing water pollution.The pollution of water bodies by dye wastewater is an important part of water pollution.The released dyes are highly toxic, carcinogenic and heterologous to the organism.We urgently need to find new ways to solve energy and environmental problems.Photocatalyst has attracted much attention because of its advantages of low cost and pollution-free solution to environmental problems [1].In recent years, the main research directions of photocatalytic materials are metal oxides [2], nitrides [3], sulfides [4], MOFs photocatalytic materials [5], photonic crystal photocatalytic materials [6], and surface plasmon resonance materials [7].Conventional photocatalytic materials have many shortcomings, for example, the recombination rate between photogenerated electrons and holes is relatively large and the band gap is relatively wide.TiO 2 only receives ultraviolet light due to its band gap [8].
As a composite metal oxide of iron and bismuth, Bi 2 Fe 4 O 9 is widely used in various fields due to its suitable band gap (about 2.1 eV) [9].Currently, it is widely used in photocatalytic hydrogen production [10], battery [11], photocatalyst [12], photodetector [13], etc. Bi 2 Fe 4 O 9 was characterized by XRD, SEM and TEM, and the samples were tested by CV curve and XPS [11].Density functional theory (DFT) based projector augmented wave (PAW) method is used to elucidate the structural, electronic, magnetic, and first order optical properties of different magnetic ordering of Bi 2 Fe 4 O 9 [14].The structural, electronic and magnetic properties of ferromagnetic phase of Bi 2 Fe 4 O 9 have been studied based on the density functional theory (DFT) [15].However, there are some defects in Bi 2 Fe 4 O 9 , which result in a high carrier recombination rate during photocatalysis, so the ability to degrade pollutants is poor.At present, the methods to improve the photocatalytic ability of Bi 2 Fe 4 O 9 include surface deposition of precious metals [16,17], the composite of suitable semiconductor materials [18], and doping of common ions [19].Bi 2 Fe 4 O 9 is a mullite structure and an antiferromagnetic material.The Nail transition temperature (TN) is about 260 K, belonging to the orthorhombic phase.The space group is the Pbam group.The lattice parameters are a = 7.965 Å, b = 8.440 Å, c = 5.944 Å [20][21][22], and Fig. 1 is the schematic diagram of Bi 2 Fe 4 O 9 crystal structure.At present, the preparation methods of single phase Bi 2 Fe 4 O 9 include the hydrothermal method and solid phase method.g-C 3 N 4 is a new non-metallic semiconductor material with a graphene like structure, which has a simple operation process and good performance and stability.Di et al. [23] prepared g-C 3 N 4 /CNT/Bi 2 Fe 4 O 9 with a ternary heterojunction structure.Introducing CNT into the ternary composite as an excellent solid electronic medium can effectively accelerate the electron migration between Bi 2 Fe 4 O 9 and g-C 3 N 4 , enabling efficient separation of photogenerated carriers, thereby greatly improving the photocatalytic degradation ability of the material.Shekardasht et al. [24] prepared the Z-type g-C 3 N 4 /RGO/Bi 2 Fe 4 O 9 nanocomposite by hydrothermal method to degrade Congo red dye.Through the photocatalytic experiment, it was found that the photocatalytic activity of the prepared composite was better than that of g-C 3 N 4 , Bi 2 Fe 4 O 9 , RGO and g-C 3 N 4 /Bi 2 Fe 4 O 9 .
In this study, g-C 3 N 4 /Bi 2 Fe 4 O 9 (5%, 10%, 15%, 20%, 25%, 30%) photocatalyst was prepared by mechanica mixing-calcination method.According to the above method, g-C 3 N 4 /Bi 2 Fe 4 O 9 was tested to study the crystal structure, morphology and size of the material, the change of carrier movement rate and the change of band gap width.The best composite ratio was obtained through the photocatalytic degradation experiment.The group with the best photocatalytic performance was selected for the capture agent experiment and cycle experiment, and the photocatalytic mechanism was comprehensively analyzed.

Preparation
Preparation of g-C 3 N 4 (calcination method): Weigh 5 g of melamine with a high-precision electronic balance, put it in a crucible with a lid, and then put it in a muffle furnace.The heating rate is set at 5°C/min, the maximum temperature is set at 550 °C, and keep it at this temperature for 5 h; With the furnace to room temperature, take out g-C 3 N 4 , fully grind g-C 3 N 4 , bag for reserve.
Preparation of Bi 2 Fe 4 O 9 (coprecipitation-hydrothermal method): weigh Fe(NO 3 ) 3 •9H 2 O and Bi(NO 3 ) 3 •5H 2 O in a beaker according to the molar ratio of n(Fe):n(Bi) = 2:1, and prepare concentrated nitric acid solution into dilute nitric acid solution with concentration of 10%.Add 15 mL of dilute nitric acid with a concentration of 10% to the beaker, and then stir for 30 min with a stirrer.Weigh 0.105 mol of KOH into a beaker, add 15 mL of deionized water into the beaker, and stir until dissolved, to prepare a KOH aqueous solution with a concentration of 7 M. Add the prepared KOH aqueous solution to the mixed solution drop by drop, and use the stirrer to continuously stir for 1 h.Put the mixed solution into the reaction kettle and seal it, then put it into the blast drying oven, and keep it at 200 °C for 6 h.Take out the sample after the reaction kettle was cooled to normal temperature.After that, it was washed with deionized water and anhydrous ethanol solution in turn, put in a vacuum drying oven and dried at 60 °C for 12 h.After fully drying, grind it with an abrasive dish to obtain the Bi 2 Fe 4 O 9 powder.
Preparation of g-C 3 N 4 /Bi 2 Fe 4 O 9 (mechanica mixingcalcination method): According to a certain mass ratio (5%, 10%, 15%, 20%, 25%, 30%), g-C 3 N 4 and Bi 2 Fe 4 O 9 were put into the grinding dish, mechanical grinding 30 min; The ground powder was put into the muffle furnace, and the muffle furnace was set to heat up at the rate of 5 °C/min, and the temperature was set to 300 °C, which was kept at this temperature for 1 h.After cooling at room temperature, take out the product, grind and bag it to obtain 5%-g-

Characterization Techniques
In this paper, the X-ray diffractometer D/max-3B of Japanese Science is used.The scanning range is 10-90°, and the radiation source is Cu-Kα (λ = 0.15418 nm).The composition, crystallinity and crystal structure of Bi 2 Fe 4 O 9 , BiOCl and their composites were analyzed and determined by correlation diffraction experiments.The samples were microscopically analyzed by FEISirion-200 scanning electron microscope, and the working voltage was 20 kV.The morphologic and dimensional characteristics of Bi 2 Fe 4 O 9 , g-C 3 N 4 and their composites were studied and analyzed, and the element components were analyzed by EDS.A small amount of samples are diluted with ethanol, then coated on conductive silicon wafers, which are pretreated by gold spraying, and then put into a test stand for testing.This paper adopts JEOL JEM-2010 transmission electron microscope with working voltage of 200 kV.The sample size and The lattice fringes of the powder catalysts could be seen by high power transmission electron microscopy.The microsamples were diluted with ethanol solution, treated with ultrasound for 30 min, absorbed 10 μL solution with pipette, dropped onto the filter paper supporting film copper net, dried and tested.In this paper, the RF-6000 fluorescence spectrophotometer of Shimadzu Company in Japan was used, and the excitation wavelength was set at 350 nm.The characterization tests of Bi 2 Fe 4 O 9 , g-C 3 N 4 and their composites were carried out.The catalyst sample was plated on the conductive surface of the FTO glass and put into the instrument for testing.Using RST5000 electrochemical workstation, the catalyst was tested by AC impedance (EIS), the interface resistance of the material was measured, and the electron transmission rate was analyzed.The working electrode was used to load the FTO glass of the sample, the reference electrode was Ag/AgCl, the Pt electrode was used to counter the electrode, and 0.5 mol/L Na 2 SO 4 solution was used as the conducting medium.The test range of frequency was set from 0.01 to 100,000 Hz, and the AC amplitude was set as 0.001 V. USB-4000 UV-visible diffusometer was used in this experiment.With the plastic plate as the base, the iron sheet with holes was placed on the plastic plate, and then the powder to be measured was laid flat in the holes.After compaction, the solid BaSO 4 was used as the reference for testing.The absorption band edge of Bi 2 Fe 4 O 9 , g-C 3 N 4 and g-C 3 N 4 /Bi 2 Fe 4 O 9 in the UV-visible region was tested, and the band gap was calculated.

Photocatalytic Experiment
Degradation test method of RhB: Prepare 10 mg/L RhB 100 mL, stir with magnetic stirrer, weigh 0.1 g of the prepared sample and add it into the dye solution.First, the sample was completely screened for 60 min to test the dark adsorption performance.10 mL solution was taken every 30 min and the absorbance was measured by 722N spectrophotometer (330-1000 nm).The wavelength of RhB dye was selected as 554 nm.The dye solution at the end of the dark adsorption treatment was placed under xenon lamp, and the photocatalytic degradation of dye was tested by xenon lamp illumination.The absorbance was measured by spectrophotometer every 30 min, and the test time was 3-4 h.The decolorization rate D of RhB degradation at different time was calculated by Eq. (1).
In Formula (1), A t is the absorbance of RhB dye solution at different time; A 0 is the initial absorbance of the RhB dye solution. (1) Capture agent test: In this project, disodium ethylenediamine tetraacetate (EDTA-2Na), tert-butyl alcohol (TBA), and benzoquinone (BQ) were used as trapping agents for vacancies (h + ), hydrl radicals (•OH), and superoxide radicals (O 2 − •), respectively.The g-C 3 N 4 and Bi 2 Fe 4 O 9 composite was investigated for the active ingredient that play a major role in the photocatalytic degradation experiments.The operation method was similar to the photocatalytic experimental method, and the catalysts were added followed by 1 mM of EDTA-2Na, TBA and BQ successively for photocatalytic degradation tests.Figure 3a shows the SEM image of Bi 2 Fe 4 O 9 , and it can be observed that Bi 2 Fe 4 O 9 is lamellar structure.Figure 3b is the SEM image of g-C 3 N 4 .g-C 3 N 4 is 14 μm, showing irregular sheets stacked together.It can be seen from Fig. 3c that a large amount of Bi 2 Fe 4 O 9 is loaded on the g-C 3 N 4 lamellar surface.Figure 3d is a local enlargement, and the contact surface of g-C 3 N 4 and Bi 2 Fe 4 O 9 can be clearly seen.

SEM and TEM Analysis
Figure 4 shows the TEM and HRTEM images of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 .
The 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 composite was characterized by TEM and HRTEM, and the composite of g-C 3 N 4 and Bi 2 Fe 4 O 9 was observed.As shown in Fig. 4a, it can be seen that Bi 2 Fe 4 O 9 is loaded on the surface of g-C 3 N 4 , the size of g-C 3 N 4 is large, and the size of Bi 2 Fe 4 O 9 is small and the number is large.Through partial enlargement of Fig. 4b, the clear contact surface of the two substances can be seen.The HRTEM diagram in Fig. 4c shows that the distance between the lattice fringes of Bi 2 Fe 4 O 9 is 0.26 nm, corresponding to the (130) crystal plane, and g-C 3 N 4 has no lattice fringes.From Fig. 6, it can be seen that the Nyquist curve radius of Bi 2 Fe 4 O 9 and g-C 3 N 4 is similar, indicating that the resistance to electron migration of the two materials is about the same.The Nyquist curve radius of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 is relatively small, indicating that the resistance to electron migration of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 is the smallest.Through the comparison of the three curves, the photogenerated carrier transfer efficiency of g-C 3 N 4 /Bi 2 Fe 4 O 9 is the highest, which indicates that the combination of the two can reduce the transmission resistance of the material interface, accelerate the electron transfer rate, and promote the photocatalytic performance of the material.As shown in Fig. 7a, the absorption band edge of Bi 2 Fe 4 O 9 is about 650 nm, and that of g-C 3 N 4 is about 476 nm.The absorption band edge of g-C 3 N 4 /Bi 2 Fe 4 O 9 composite is larger than that of the two materials, and the absorption band edge of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 is the largest, about 690 nm.As shown in Fig. 7b, the band gap width of Bi 2 Fe 4 O 9 is 1.9 eV, the band gap width of g-C 3 N 4 is 2.6 eV, and the band gap width of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 is 1.79 eV.With the increase of the g-C 3 N 4 composite ratio, the band gap width of the material decreases gradually, and the kinetic energy required for the electron level transition decreases, which improves the carrier separation efficiency, and also improves the ability of the material to degrade dyes.photocatalyst surface, which was conducive to the photolysis reaction between the free radicals adsorbed on the photocatalyst and the target organic matter.The combination of g-C 3 N 4 and Bi 2 Fe 4 O 9 can improve the microstructure of the material, helping to make full use of light for photocatalysis.It can also be concluded that adsorption is the prerequisite of photocatalytic reaction, and appropriate adsorption activity can promote the photocatalytic efficiency of the material [27,28].The decolorization rate of g-C 3 N 4 and Bi 2 Fe 4 O 9 is 47.57% and 45.7% respectively at 60-240 min as the photocatalytic stage.The photocatalytic decoloration rate of different mass ratios of g-C 3 N 4 /Bi 2 Fe 4 O 9 is higher than that of Bi 2 Fe 4 O 9 , and the decoloration rate of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 is the best, which is 87.59% at 240 min.It is the combination with g-C 3 N 4 that reduces the electron and hole recombination efficiency of the material, accelerates the electron transfer rate, and improves the photocatalytic degradation performance of the material, so the catalytic effect is very good [29].

Photocatalytic Performance Analysis
It has been proved by a large number of documents that Langmuir-Hinshelwood first-order kinetic model [30] is suitable for most photocatalytic degradation systems.Therefore, the first-order kinetic model can be used to conduct quantitative studies on g-C 3 N 4 , Bi 2 Fe 4 O 9 and g-C 3 N 4 /Bi 2 Fe 4 O 9 .In the Eq. ( 2), C 0 is the initial concentration of the RhB solution; C t is the concentration of the RhB solution at the moment t (min) and k 1 is the quasi first-order reaction rate constant (min −1 ) [31].
(2)  As shown in Fig. 9, RhB has an obvious absorption peak at 554 nm.In A 0 -A 2 stage, the height of the characteristic absorption peak decreases, but the position of the peak does not shift.With the gradual progress of the degradation reaction, the height of the characteristic absorption peak of RhB gradually decreased, and there was no shift, indicating that 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 directly degraded RhB into H 2 O and CO 2 during the degradation process, without producing secondary products.As shown in Fig. 10, when EDTA-2Na was added, the photocatalytic degradation rate did not change much, indicating that h + has no effect on the photocatalytic degradation rate of RhB.When TBA and BQ were added to the photocatalytic system, the catalytic degradation rate decreased to 42.  5)-( 9).

Capture Agent Test
(3)   Select 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 with the best photocatalytic effect to carry out the cycle experiment, and repeat the experiment for four times in total.Since the amount of photocatalyst will be lost when the catalyst is washed and dried, the photocatalytic performance will be affected to a certain extent.As shown in Fig. 12, after four cycle tests, the decoloration rate of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 for RhB is 65.65%, and the photocatalytic decoloration rate decreases by 21.94%, which indicates that 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 has good stability.

Conclusions
In this study, g-C 3 N 4 /Bi 2 Fe 4 O 9 catalyst was prepared by mechanical mixing-calcination method and coprecipitation-hydrothermal method.Through XRD, SEM and TEM analysis, the composite is composed of two basic materials, Bi Funding The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Declarations
Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figure 2
Figure 2 shows the XRD diagrams of Bi 2 Fe 4 O 9 , g-C 3 N 4 and g-C 3 N 4 /Bi 2 Fe 4 O 9 with different mass ratios.As shown in Fig. 2, g-C 3 N 4 is at 2θ = 26.5°has a characteristic diffraction peak corresponding to the (002) crystal plane of the g-C 3 N 4 standard card (PDF87-1526).When the proportion of g-C 3 N 4 is 5%, the characteristic peak is basically consistent with that of Bi 2 Fe 4 O 9 due to the small amount of compound.With the increasing of the ratio of g-C 3 N 4 , the diffraction peaks of g-C 3 N 4 become more and more obvious.When the composite ratio of g-C 3 N 4 is 10-30%, the characteristic diffraction peak appears at 2θ = 26.5°,corresponding to the (002) crystal plane of g-C 3 N 4 , and the intensity of the diffraction peaks at 2θ = 28.19°,29.79° gradually decreases, corresponding to the (121), (002) crystal planes of Bi 2 Fe 4 O 9 .In Fig. 2, g-C 3 N 4 /Bi 2 Fe 4 O 9 shows the characteristic peaks of the two

Figure 3
Figure 3 shows the SEM images of Bi 2 Fe 4 O 9 , g-C 3 N 4 and 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 and local enlargement.Figure3ashows the SEM image of Bi 2 Fe 4 O 9 , and it can be observed that Bi 2 Fe 4 O 9 is lamellar structure.Figure3bis the SEM image of g-C 3 N 4 .g-C 3 N 4 is 14 μm, showing irregular sheets stacked together.It can be seen from Fig.3cthat a large amount of Bi 2 Fe 4 O 9 is loaded on the g-C 3 N 4 lamellar surface.Figure3dis a local enlargement, and the contact surface of g-C 3 N 4 and Bi 2 Fe 4 O 9 can be clearly seen.Figure4shows the TEM and HRTEM images of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 .The 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 composite was characterized by TEM and HRTEM, and the composite of g-C 3 N 4 and Bi 2 Fe 4 O 9 was observed.As shown in Fig.4a, it can be seen that Bi 2 Fe 4 O 9 is loaded on the surface of g-C 3 N 4 , the size

Figure 5 Fig. 3
Figure 5 shows the photoluminescence spectrum of g-C 3 N 4 / Bi 2 Fe 4 O 9 with different mass ratio.As shown in Fig. 5, the luminescence peak of g-C 3 N 4 is about 470 nm, that of Bi 2 Fe 4 O 9 is about 469 nm, and the luminescence peaks of g-C 3 N 4 /Bi 2 Fe 4 O 9 are smaller than that of Bi 2 Fe 4 O 9 .In visible light, the addition of a small amount of g-C 3 N 4 improves the photoelectrochemical reaction of the material.Appropriate g-C 3 N 4 (0-25%) compound on Bi 2 Fe 4 O 9 can inhibit the recombination rate of photogenerated carriers and improve the separation and transfer of charge in visible light.With the increase of g-C 3 N 4 content,

Figure 7
Figure 7 shows the UV-Vis DRS spectrums and the forbidden band width curves of the g-C 3 N 4 /Bi 2 Fe 4 O 9 composite material.As shown in Fig.7a, the absorption band edge of Bi 2 Fe 4 O 9 is about 650 nm, and that of g-C 3 N 4 is about 476 nm.The absorption band edge of g-C 3 N 4 /Bi 2 Fe 4 O 9 composite is larger than that of the two materials, and the absorption band edge of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 is the largest, about 690 nm.As shown in Fig.7b, the band gap width of Bi 2 Fe 4 O 9 is 1.9 eV, the band gap width of g-C 3 N 4 is 2.6 eV, and the band gap width of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 is 1.79 eV.With the increase of the g-C 3 N 4 composite ratio, the band gap width of the material decreases gradually, and the kinetic energy required for the electron level transition decreases, which improves the carrier separation efficiency, and also improves the ability of the material to degrade dyes.

Figure 8
Figure 8 shows the change curve and kinetic analysis of the decolorization rate of g-C 3 N 4 /Bi 2 Fe 4 O 9 composite for the degradation of 10 mg/L RhB.As shown in Fig. 8a, it belongs to the dark adsorption stage during 0-60 min, and the adsorption rates of g-C 3 N 4 and Bi 2 Fe 4 O 9 are basically the same.The adsorption rate of g-C 3 N 4 /Bi 2 Fe 4 O 9 with a composite ratio of 15-30% is higher than that of Bi 2 Fe 4 O 9 , and the highest adsorption rate is 30% g-C 3 N 4 /Bi 2 Fe 4 O 9 , about 35.04%.According to the literatures, the nature of the surface of the material, number of active sites, morphology, and cationic/anionic characteristics of the substrate, influence the absorptive capacity of the materials and favor the degradation of the dye due to their proximity to the active sites distributed on the surface of the material.g-C 3 N 4 / Bi 2 Fe 4 O 9 significantly improved the adsorption of RhB on the

Figure 9
Figure 9 shows the UV absorption spectrum of the solution in the process of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 photocatalytic degradation of RhB.As shown in Fig.9, RhB has an obvious absorption peak at 554 nm.In A 0 -A 2 stage, the height of the characteristic absorption peak decreases, but the position of the peak does not shift.With the gradual progress of the degradation reaction, the height of the characteristic absorption peak of RhB gradually decreased, and there was no shift, indicating that 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 directly degraded RhB into H 2 O and CO 2 during the degradation process, without producing secondary products.

Figure 10
Figure 10 is a graph showing the decolorization rate curve of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 on RhB after adding different capture agents.As shown in Fig.10, when EDTA-2Na was added, the photocatalytic degradation rate did not change much, indicating that h + has no effect on the photocatalytic degradation rate of RhB.When TBA and BQ were added to the photocatalytic system, the catalytic degradation rate decreased to 42.3% and 50% respectively at 240 min.Based on the above results, it

2 −
Figure 10 is a graph showing the decolorization rate curve of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 on RhB after adding different capture agents.As shown in Fig.10, when EDTA-2Na was added, the photocatalytic degradation rate did not change much, indicating that h + has no effect on the photocatalytic degradation rate of RhB.When TBA and BQ were added to the photocatalytic system, the catalytic degradation rate decreased to 42.3% and 50% respectively at 240 min.Based on the above results, it

Figure 12
Figure 12 is a cycle histogram of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 photocatalytic degradation of RhB solution.Select 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 with the best photocatalytic effect to carry out the cycle experiment, and repeat the experiment for four times in total.Since the amount of photocatalyst will be lost when the catalyst is washed and dried, the photocatalytic performance will be affected to a certain extent.As shown in Fig.12, after four cycle tests, the decoloration rate of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 for RhB is 65.65%, and the photocatalytic decoloration rate decreases by 21.94%, which indicates that 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 has good stability.

2
Fe 4 O 9 is loaded on the surface of g-C 3 N 4 .After the heterojunction structure is formed, the electron and hole recombination efficiency of g-C 3 N 4 /Bi 2 Fe 4 O 9 decreases and the impedance decreases, which enhances the photogenerated carrier mobility efficiency and contributes to the pollutant degradation performance of the material.At 240 min, the decolorization rate of 25% g-C 3 N 4 /Bi 2 Fe 4 O 9 reaches 87.59%, and the reaction rate constant is 0.00674 min −1 , which is 2.53 and 2.32 times that of Bi 2 Fe 4 O 9 and g-C 3 N 4 .O 2 − • and •OH were the main active components in the degradation process.The catalytic activity is still relatively stable after four cycles of experiments.Author Contributions SQ: funding acquisition, investigation, project administration, resources, supervision.LG: conceptualization, data curation, formal analysis, methodology, software, visualization, writing-original draft.RZ: supervision, validation, writing-review & editing.SW: supervision, validation, writing-review & editing.KZ: supervision, validation, writing-review & editing.